For avoiding collisions, you only really need to remember a few things:
1 - The driver in front has the right to take whichever line he (or she) wishes to take, and the driver behind is responsible for avoiding a collision, that means he also needs to account for early braking on the part of the driver who is in front of him.
2 - When overtaking in a turn, the driver in front needs to be aware of the other driver's intentions, and the right of way will be determined by significant overlap. A driver has overlap when the front of his car is in front of the other driver's HEAD, not his front bumper. If that condition is met, the driver being overtaken must then lift off ever so slightly, just so he can get out of the corner right behind the other car and remain battling normally.
3 - Blocking is okay if done once before each corner, as the previous poster said. If done wisely (but safely, otherwise you'll just clip the front bumper of the trailing car..), the driver who's attempting to make an overtaking maneuvre will need to lift off and will lose time.
4 - Abrupt reductions of speed are definitely not recommended, even when being lapped. The car being lapped must lift off slightly and remain in one place until he is lapped, because the driver behind has a better view of what is happening. Once he's lapped, he may continue racing normally. If he loses speed too suddenly, there's a chance the driver who's lapping won't have time to react. If this happens in a turn, the driver being lapped has to take a line that is not the ideal one, and remain in it until he can continue racing normally.
Sounds like too much to keep in mind, but they're really simple rules that could be summed up in one or two lines.
